A lithographic printing plate precursor is disclosed which comprises a support, a photopolymerizable image recording layer and an overcoat which comprises a low-molecular radical inhibitor. After image-wise exposure, the plate is heated whereby the radical inhibitor diffuses from the overcoat to the image recording layer, resulting in an increase of the daylight stability of the exposed and heated precursor. Such plate is especially suitable for on-press processing.

An object of the present invention is to provide a color developing composition which develops colors in a high density and does not significantly discolor when aged, a lithographic printing plate precursor which has excellent plate-inspecting properties by means of color development and is capable of maintaining strong color development even when aged after color development, a plate making method for a lithographic printing plate in which the lithographic printing plate precursor is used, and a new compound that can be preferably used as a color developer.

The color developing composition of the present invention includes a compound represented by Formula 1. The compound in the present invention is represented by Formula 1. In Formula 1, R.sup.1 represents a group in which an R.sup.1O bond is cleaved by heat or exposure to infrared rays.

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Dry, ablation-type, nitrocellulose-containing lithographic printing members include dual adjacent imaging layers, both including an absorber and at least one containing a binder (which may include or consist essentially of a melamine resin). The absorber of the nitrocellulose-containing layer is a pigment and this layer contains no absorbing dye, while the absorber of the other imaging layer includes or consists essentially of a dye.

A recording medium comprises an oleophilic substrate and, thereover, a topmost oleophobic layer comprising a cured polymeric silicone matrix that consists essentially of the addition-cure reaction product of a vinyl-functional polydialkylsiloxane and a silane cross-linking agent. The vinyl-functional polydialkylsiloxane has a molecular weight ranging from 30,000 to 75,000 or 110,000 to 130,000. If the molecular weight of the vinyl-functional polydialkylsiloxane is within the range of 30,000 to 75,000, the molar ratio of silane groups to vinyl groups is within the range of about 11:1 to about 25:1; and if the molecular weight of the vinyl-functional polydialkylsiloxane is within the range of 110,000 to 130,000, the molar ratio of silane groups to vinyl groups is from about 5:1 to about 27:1. The recording medium may be used as a lithographic printing plate.

An on-machine development type planographic printing plate precursor has a support and an image-recording layer on the support. The image-recording layer contains an initiator, an infrared absorber capable of donating electrons to the initiator, and a color-developing substance precursor. The image-recording layer can form an image by exposure to infrared laser, and in a case where the image-recording layer is exposed to an infrared laser with a wavelength of 830 nm at an energy density of 110 mJ/cm.sup.2, a brightness change L of the image-recording layer before and after the exposure is 3.0 or more. A method for preparing a planographic printing plate and a planographic printing method employ the on-machine development type planographic printing plate precursor.

In ablation-type printing plates involving silicone acrylate top layers, curing at high oxygen levels not only substantially reduces or eliminates toning, but does not adversely affect plate durability or printing performance.

A lithographic printing plate precursor is disclosed which includes a support and a coating comprising a photopolymerisable layer including a polymerisable compound, an initiator and a pH sensitive colorant precursor and a protective overcoat layer provided above the photopolymerisable layer including a heat sensitive color-forming IR dye, which upon exposure to heat and/or light forms a print-out image which remains stable or increases during storage of the precursor in light or dark environment.

An object of the present invention is to provide an on-press development-type planographic printing plate precursor which suppresses color-forming defect after a lapse of time and has excellent visibility. Another object of the present invention is to provide a manufacturing method of a planographic printing plate and a printing method. The planographic printing plate precursor of the present invention is an on-press development-type planographic printing plate precursor including a support and an image-recording layer, in which the support has an aluminum alloy plate and an anodic oxide film disposed on the aluminum alloy plate, a density of intermetallic compounds having a major axis of 2.0 m or more on a surface of the support on a side opposite to the image-recording layer is 1,000 pieces/mm.sup.2 or less, the image-recording layer contains an infrared absorber and an acid color-forming agent, and a color-forming factor of the acid color-forming agent, which is determined by a predetermined measuring method, is 20,000 or more.

Provided are an on-press development type lithographic printing plate precursor including: a support; and an image-recording layer on the support, in which the image-recording layer contains an electron-accepting polymerization initiator, an electron-donating polymerization initiator, an infrared absorber, and a salt compound A, and an HOMO value of an anion constituting a salt in the salt compound A is 6.2 eV or more and 4.4 eV or less, and a method of preparing a lithographic printing plate or a lithographic printing method using the on-press development type lithographic printing plate precursor.

A lithographic printing plate precursor has a support and an image-recording layer on the support, in which the image-recording layer contains an infrared absorber, a polymerization initiator, and a polymer A. The weight-average molecular weight of the polymer A is more than 15,000 and 150,000 or less, and the ethylenically unsaturated bond valence of the polymer A is 3.0 mmol/g or more.